The present invention relates to a data transfer system for the optical transmission of data using optical waveguides, wherein the transmitter and/or receiver must be displaced along an optical waveguide or must be positioned in different positions. Such data transfer systems are employed, for example, in a linear configuration in crane installations or other conveying systems for the transmission of data between the mobile crane and a stationary control unit. Another field of application of these data transfer systems in a circular configuration is the transmission between parts rotatable relative to each other, such as in a computer tomograph between the rotor that supports the X-ray tube and the detector, and a stationary analyzing unit that processes and displays the video data.
In common transfer systems operating on the basis of optical waveguides, light is coupled into one end of the optical waveguide and is then passed through the optical waveguide up to the latter's other end where it is analyzed again by an appropriate receiver. Based on this system, a great number of different variants have become known, which permits the simultaneous transmission of several wavelengths by means of filters or even the transfer to several different sites, for example by way of Y-couplers. Such systems are, however, not suitable for coupling or decoupling signals into or out of a light-guide fiber at any position whatsoever. To this end, various other technologies are known.
U.S. Pat. No. 4,962,986 discloses a system in which a coupling medium, whose refractive index is higher than the index in the environment, is contacted directly with the fiber core for coupling and decoupling light into and out of light-guiding fibers. As a result, the light conveyed in the fiber is deflected into the coupling medium. This arrangement presents the decisive disadvantage that the coupling medium must be in direct connection with the fiber core. Hence, this system is suitable for application almost exclusively for coupling at predetermined invariable positions. Such a system, however, is hardly applicable for arrangements in which the transmitter and the receiver are mobile relative to each other because here, the coupling medium must slide at a high speed along the fiber core, which is mostly very thin and highly sensitive.
A more expedient device is described in U.S. Pat. No. 5,297,225. There, light, which is coupled in from the outside through notches formed on the outside in the light-conducting medium, is deflected by reflection at such angles that it can be guided in the medium. Such a transmission device is suitable for sensible application when coupling is to take place at invariably predetermined locations. In principle, it is also applicable for the transmission between mobile units because the light is coupled in or coupled out without contact. When, as a matter of fact, a fairly long path of movement is required, which is the case in crane installations or also rotary transformer systems, the great number of notches along the light-guiding medium results in an unacceptable strong attenuation.
In an approach to eliminate the disadvantage of strong attenuation, the fluorescence system has been developed, which is described in the PCT document WO 95/35605. Here, the light-guiding fiber is doped with a fluorescent dye. Light incident from the outside is absorbed by the molecules of the fluorescent dye, which are then stimulated to emit light. The emission of light takes place in all directions, in a manner similar to a spherical light source. Hence, light is generated inside the light-guiding fiber, of which a slight fraction is also within the acceptance angle of this fiber and can hence by passed on the fiber. The advantage of this method resides in the conversion of the wavelength by the molecules of the fluorescent dye. The energy level in the light emitted by the molecules of the fluorescent dye is thus fundamentally lower than the energy level of the light absorbed by the molecules. The emitted light hence presents a longer wavelength. The energy of the light emitted by a molecule of the fluorescent dye is not sufficient for being absorbed in further molecules of the same kind in the fluorescent dye and for causing, in its turn, the emission of fluorescent light again. The light-guiding fiber doped with molecules of the florescent dye hence displays a weaker attenuation for the light generated by the fluorescence effect. As a result, transfer systems of a great length or of a wide diameter can therefore be implemented in an efficient manner. This system now presents the substantial disadvantage that the fluorescence effect is not stopped spontaneously when the energy supply by the stimulating light is stopped but rather undergoes an exponential decay. This leads to a limitation of the transferred signals in terms of speed or bandwidth, respectively. The best fibers with fluorescent dyes, which have been tested in laboratory tests, display time constants in the order of some nano seconds and are hence suitable for application only up to several 100 Mbaud but by no means for the Gbaud range.
This limitation of the bandwidth is avoided by the transfer system represented in the PCT document WO 98/00936. In a manner similar to the aforedescribed system, here light is coupled from the outside into a light-guiding fiber that has been doped with dyes. Here, however, it is the stimulated emission of a material whose electron array can be inverted by energetic stimulation, rather than the fluorescence effect, which is utilized. The disadvantage of this arrangement is the high expenditure in engineering terms, for example in wavelength multiplexers and pumping light sources. As a result, such arrangements are not yet suitable for the widerange series application on an industrial scale.
Another method is based on the coupling of light via an optical grating that is applied, for example, by photo-refractive materials inside a glass fiber (as is published in U.S. Pat. No. 4,749,248) or in the form of an envelope on a glass fiber (as is published in the PCT document WO 99/04309).
In the statements presented in the following, reference is made to the term “optical waveguide”. This term relates to the preferred embodiment because light must be guided in an attenuation-free manner only on one light guide particularly in the case of fairly long distances. The subject matter of the invention is, of course, applicable also to all other kinds of light guides with the same effects.